Conventionally, in optical transport networks (OTN) using optical fibers as transmission media, a ring network in which a plurality of nodes are connected in a ring shape has become common. This topology of a network can be expanded by interconnecting rings; however, on the occurrence of a node or line failure, to curb the influence of the failure, 8-shaped expansion by connection of single nodes is not usually performed. Usually, for example, in view of the ring protection recommendation G.873.2 defined by International Telecommunication Union-Telecommunication standardization sector (ITU-T), an expansion method which is highly redundant with respect to failure is adopted. Examples of the method include a shared link that connects rings is formed using two nodes and an interconnection link that further improves the redundancy using four nodes arranged in a rectangle shape.
[Patent document 1] Japanese Laid-open Patent Publication No. 11-331227
[Patent document 2] Japanese Laid-open Patent Publication No. 2002-009802
Out of the above-mentioned two types of expansion methods (links), especially, the latter interconnection link is standardized, for example, in ITU-T G.842, and is capable of expansion of a ring network while achieving Bi-directional Line Switched Ring (BLSR)-based protection. However, this method has a problem that nodes other than connection nodes also have to create a table for path line or control traffic to an adjacent ring. This problem can be addressed by, for example, in four connection nodes, by adopting a configuration that links directly connecting diagonally opposite nodes are formed (a so-called “X-shaped configuration”); however, in the above-mentioned ITU-T G.842, a new issue arises. That is, when a network adopts the configuration, there is an increase in the number of branches of a selector or the number of nodes in the network, or the nodes have to include several types of selectors. As a result, the state management of a connecting part (connection nodes or links between nodes) between rings becomes complicated.